The software used in the latest attacks is called WannaCry, or Wanna Decryptor, and exploits a vulnerability in the Windows operating system.
It allows the malware to automatically spread across networks, so it can quickly infect large numbers of machines at the same organization.
The Cyber extortionists tricked victims into opening malicious attachments to spam emails that appeared to contain invoices, job offers, security warnings and other legitimate files.
The ransomware encrypted data on the computers, demanding payments of $300 to $600 to restore access.
Security researchers said they observed some victims paying via the digital currency bitcoin, though they did not know what percent had given in to the extortionists.
Spain took steps to protect critical infrastructure in response to the attack.
Authorities said they were communicating with more than 100 energy, transportation, telecommunications and financial services providers about the attack.
Telefonica said the attack was limited to some computers on an internal network and had not affected clients or services.
In the US, the effect of the hack did not appear to be widespread, at least initially.
Hacking group Shadow Brokers reportedly released the malware last month, after claiming to have discovered the flaw from the US National Security Agency.
Up to 99 countries may have been affected by the ransomware cyber attack that has struck the NHS, according to some experts.
It is believed to be the biggest attack of its kind ever recorded.
Russia appeared to be the hardest hit nation, with its interior and emergencies ministries and biggest bank, Sberbank, saying they were targeted.
The interior ministry said on its website around 1,000 computers had been infected but it had localized the virus.
Spain, Ukraine, and India were also severely affected, according to researchers from the Kaspersky Lab.
By the group’s count, the malware struck at least 74 countries. However, researchers with security software maker Avast said they had observed 57,000 infections in 99 countries, also citing Taiwan among the top targets.
Cyber security expert Varun Badwhar said the attack gave a glimpse of what a “cyber-apocalypse” would look like.